Precision oven-controlled crystal oscillator

ABSTRACT

A precision oven-controlled crystal oscillator (OCXO) uses an adjustment feedback signal that, when mixed with a reference signal from a stable reference oscillator, accurately controls the generation of an output signal from a voltage controlled crystal oscillator (VCXO). An OCXO according to the invention has high stability and high accuracy. The digital OXCO can be manufactured at low cost, and is particularly beneficial for Code Division Multiple Access (CDMA) base station applications in cellular communication networks an the like.

CROSS REFERENCE TO RELATED DOCUMENTS

This application claims the benefit claimed under 35 U.S.C. § 119(e) ofco-pending provisional application, Ser. No. 60/256,009, filed Dec. 15,2000, entitled “DIGITAL OVENIZED CRYSTAL OSCILLATOR” by John C. Ho, etal. A copy of the provisional application is included herewith, and theprovisional application is incorporated by reference in the entiretyherein.

BACKGROUND OF THE INVENTION

The invention relates generally to oscillators and, more particularly,to oven-controlled or “ovenized” quartz crystal oscillators or OCXOs.

Oscillators are used to generate frequencies for applications varyingfrom relatively unsophisticated applications for wristwatches and thelike, to such extremely sophisticated applications as timing systems forspace navigational systems. Most commonly, quartz crystals composed ofSiO₂ are used in oscillators, although certain highly accurate frequencystandards can be configured using an atomic reference source, such ascesium or rubidium.

Precision OCXOs, that are both stable and accurate, are highly desirablefor use in many applications. Currently, for example, both stable andaccurate OCXOs are sought for use in the design of base stations forcellular, PCS (personal communication system), and wireless local loop(WLL) systems that connect subscribers to a public switched telephonenetwork. OCXOs can be used successfully, for instance, in the transmitand receive functions or in the clocks of CDMA (Code Division MultipleAccess) base stations.

The stability of a crystal is rated according to the extent to which thecrystal's inherent instabilities can be compensated. Significantcontributors to the assessment of stability are the degree to which thefrequency of the crystal changes with temperature, and the degree towhich the frequency changes over the long-term, i.e., the agingcharacteristic of the crystal. Typically, the accuracy per year of OXCOis on the order 1×10⁻⁸, and desired stability over a wide range ofenvironmental conditions is better than 1×10⁻¹⁰. These conditionsinclude operating temperature, humidity, supply voltage variations,repeatability, frequency-setting ability, and frequency drift over longperiods of time. (Frequency “drift” is distinguishable from “aging,”insofar as the aging characteristic of a crystal is defined withreference to internal changes in oscillator when external factors, suchas the environment or the power supply voltage, are constant.)

In an OCXO, the crystal and associated components, the latter of whichmight also be sensitive to temperature, are enclosed in an oven with astable temperature. The temperature is kept constant by adjusting theamount of power supplied to the oven whenever the ambient temperature inthe oven begins to change. The oven temperature selected is one at whichthe slope of the frequency vs. temperature curve for the crystal iszero. The oven thus minimizes the degree to which the frequency of theoscillator will vary with variations in temperature.

The realization of an OCXO typically requires (1) a reference element(e.g., a quartz crystal); (2) associated circuitry for frequencygeneration or synthesis; (3) a frequency tuning element or elements; (4)a thermal control system for the oven; and (5) an output bufferamplifier so that the signal output of the OCXO can be utilized.

In order to achieve an OCXO with a desired accuracy and stability, theprecision of the reference element has always been of greatsignificance. Unfortunately, the requirement for a precise referenceelement has limited the yield of crystal production and has kept thecost of creating OCXOs high. This is because the precision of thefrequency of a crystal is affected by a great number of factors in themanufacturing process, such as the thickness of the cut of the crystalwafer, the angle of the cut, and imperfections or scratches on thecrystal. The oscillator circuitry sensitivity to the frequency of thereference element likewise has contributed to manufacturing obstacles tolarge-scale and cost-effective OCXO production. Typical frequency-tuningcomponents, such as inductors, capacitors and varactor diodes, aresensitive to environmental conditions, such as temperature, andrepeatability and tolerance drift of these components over time must betaken into account in a typical OCXO design. The thermal control systemfor the oven had to be capable of achieving very accurate temperaturesettings adjusted for the characteristics of the particular referenceelement used.

Accordingly, those concerned with the design and manufacture of OCXO'shave long recognized the need for an OCXO which can manufactured withgood yields in fairly large quantities and for reasonable cost. Thepresent invention fulfills this need.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides anoscillator and method for realizing an oscillator that is a precisionoscillator with desirable accuracy and stability over a wide range ofenvironmental conditions, even while using a reference element with afrequency that is not as precise as has been necessary in the past.According to the present invention, an accurate and stable (better than1×10⁻¹⁰/day) OCXO can be implemented using a reference element cut froma quartz bar to a thickness corresponding to, for example, 5.0033 or5.0049 MHz (as opposed to, for example to 5.0000 MHz), and which hasbeen manufactured without an especially precise cutting angle asotherwise would be required to achieve precise operating temperaturecharacteristics. The OCXO according to the invention also is notdependent on reactive components, such as capacitors and the like, totune and set the desired output frequency. Thus, by eliminating thenecessity for cost-driving features normally associated with themanufacture of a precise reference element, the present inventionresults in a very precise OCXO that is reproducible and relatively easyto manufacture at reasonable cost.

The design of the OCXO according to the invention provides a stablereference source that is not wholly dependent on the precision of thereference element. More particularly, and by way of example and notnecessarily by way of limitation, the present invention provides an OCXOcharacterized by a voltage-controlled oscillator (VCXO) which isconfigured to provide a desired predetermined output frequency, forexample, 15 MHz. The VCXO frequency also is used by a high resolutionfrequency synthesizer to generate a VCXO adjustment frequency which,when added to the VCXO output frequency, will correspond to thefrequency of the reference element or a multiple thereof. The sum of theadjustment frequency and the VCXO output frequency is then mixed withthe reference frequency, or a multiple thereof, to create asubstantially zero-beat feedback signal that is introduced into aphase-locked loop including the VCXO. The feedback signal thus locks theVCXO to the desired frequency. The high resolution frequency synthesizerthus insures that the VCXO output frequency is maintained as stable asthe frequency of the reference element, but the accuracy of the outputof the VCXO nevertheless will not be dependent on the precision of thereference element.

In a presently preferred embodiment, the VCXO adjustment frequency, isgenerated from the VCXO frequency using a direct digital synthesizer orDDS. In a DDS, adding circuitry or a phase accumulator is used toaccumulate phase at a rate dependent upon the value of the frequencyselected. The phase value is used to address some type of read-onlymemory (ROM), which stores discrete values of the sine function. Thedigital output of the read-only memory is converted to a sine wave by adigital-to-analog (D/A) converter. The sine wave is then low-passfiltered to remove such elements as the clock frequency and glitches dueto the D/A conversion. However, it should be appreciated by one skilledin the art that other techniques for high resolution synthesis of a VCXOadjustment frequency from the VCXO frequency are possible using anymethod known in the art.

In accordance with a preferred embodiment, the VCXO RF output frequency(e.g., 15 MHz) is first applied to a buffer amplifier, and the outputsignal from the buffer amplifier is applied simultaneously (1) toanother amplifier which precedes the input of the DDS, and (2) to afirst mixer. The DDS synthesizes a signal, which has been predeterminedto serve as an adjustment frequency for the VCXO. A divider preferablyis used to divide down the signal from the DDS (e.g., to 10 kHz). Theoutput of the divider is input to a first mixer, together with the VCXOoutput frequency. The mixer sums the two signals that are input to it(e.g., 15 MHz+10 kHz=15.01 MHz). The result of the first mixer then isinput into a second mixer. The other input to the second mixer is thereference element frequency (e.g., 5.0033 MHz). The second mixerfunctions to multiply the reference element frequency by three and thensubtract the result from the result of the first mixer (e.g.,(3)(5.0033)−15.01=15.0099−15.01=0.0001). This substantially zero-beatsignal from the second mixer is used to lock the VCXO, via aphase-locked loop, to maintain the VCXO output frequency. Given that thefrequency of the VCXO output frequency is adjustable via the feedbacksignal derived from the high resolution DDS, the precision of thefrequency of the reference element is not as critical to the stabilityof the oscillator as it would otherwise have been in prior art OCXOdesigns.

In some preferred embodiments of the invention, a microprocessor may beprovided to generate various control signals for, e.g., the DDS toaccomplish fine and coarse adjustments of the adjustable VCXO outputfrequency signals. For example, the OCXO may include ananalog-to-digital (A/D) converter for generating electronic frequencycontrol (EFC) signals. These EFC signals may be provided to the DDS foradjustment of the adjustable VCXO output frequency signals.Alternatively, a digital interface port may be provided to permitdigital system interface controls to the DDS for frequency adjustment.

In other preferred embodiments, a temperature sensor preferably may beincluded for generating temperature control signals or signals to adjustthe synthesizer frequency to accommodate the effect of temperaturevariations on frequency. The oven structure preferably might one with athermal gain greater than 100,000, such as can be accomplished by usinga zero-temperature gradient outer oven surrounding an inner oven.

Hence, the present invention satisfies a long-existing need for anoscillator with high stability and accuracy, which can be manufacturedat reasonable cost with high yields.

These and other objects and advantages of the invention will becomeapparent from the following, more detailed description, when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a circuit configured and operating inaccordance with one preferred of an oscillator (OCXO) of the presentinvention.

FIG. 2 is a block diagram of a circuit configured and operating inaccordance with another preferred embodiment of an OCXO of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the FIG. 1, a precision digital oven-controlled crystaloscillator 10 is illustrated. The OCXO includes a stable referenceoscillator 32, which, in this example, generates a frequency of 5.0033MHz, and a voltage-controlled crystal oscillator (VCXO) 38 which, inthis example, has a desired or predetermined radio frequency (RF) outputfrequency of 15 MHz. For the stable reference oscillator 32, afifth-overtone quartz crystal has been used with success.

In addition to being made available for use by other circuitry via a RFamplifier 42, the output frequency of the VCXO 38 also is applied to adirect digital synthesizer (DDS) 16, via first a buffer amplifier 40 anda second amplifier 18. In a preferred embodiment, the DDS 16 is a 32-bitDDS with a resolution of 2³². In another preferred embodiment, a 48-bitDDS is used.

Based on instructions from a microprocessor (μP) 12, the DDS 16synthesizes a frequency corresponding to the frequency of the VCXO 38.More specifically, the microprocessor 12 is programmed with a specificcontrol word that tells the DDS which frequency to synthesize. The samecontrol word is stored in the memory 14, which preferably iselectrically-erasable programmable read-only memory or (EEPROM or E²P).

The operation of a typical DDS 16 will now be generally described. Thecontrol word is a digital phase increment word that is stored in thej-bit frequency register of the DDS 16 phase accumulator. The phaseaccumulator also includes a j-bit full adder and a phase register. Oneach clock pulse, the data in the frequency register is added to thedata already in the phase register. The digital phase increment wordcorresponds to a phase angle step that is added to the previous value ateach 1/ƒ_(clk) seconds to create a linearly increasing digital value.The phase value is created by reason of the modulo 2^(j) overflowingcharacteristic of a j-bit phase accumulator. The rate of the overflowsis the output frequency of the phase accumulator. The read-only memoryis a sine look-up table, whereby digital phase information can beconverted into the values of a sine wave. The output of the memory isinput to a digital-to-analog (D/A) converter, and a quantitized analogsine wave results as an output. A filter after the D/A converter removeshigh frequency sampling components, such that a pure sine wave output isapproximated.

The output of the DDS 16 is input to a divider 26, which in the exampleillustrated in FIG. 1 is a 2⁸ divider, and the resultant frequency afterthe divider is 10 kHz. The frequency output from the divider 26 (e.g.,10 kHz) is introduced into a first mixer 28, into which the actual VCXO38 output frequency (e.g., 15 MHz) is also input. The first mixer 28functions to add the VCXO 38 output signal to the divider 26 outputsignal. In the example, the output of the first mixer 28 is 15 MHz+10kHz=15.01 MHz.

The output of the first mixer 28 is next input to a second mixer 30,which also receives, via an amplifier 34, the output of the stablereference oscillator 32 (e.g., 5.0033 MHz). The second mixer performsthe function 3ƒ₁−ƒ₂, where ƒ₁ is the output of the stable referenceoscillator 32 and ƒ₂ is the output of the 2⁸ divider following the DDS16. Thus, in the example, the output of the second mixer is 3(5.0033MHz)−15.01 MHz, or 15.0099 MHz−15.01 MHz=0.0001 MHz. The result of thesecond mixer 30 is amplified by amplifier 36 and input into the VCXO 38in a phase-locked loop configuration. The essentially zero-beat feedbacksignal from the second mixer 30 closes the phase-locked loop and locksthe VCXO 38 to the desired output frequency.

In the preferred embodiment in which a 48-bit DDS is used, and asillustrated in FIG. 2, the feedback signal for the phase-locked loop canbe derived using only a single mixer. In this embodiment, a divider ormultiplier 26, the choice of which is dependent upon the frequencyvalues of the stable reference oscillator 32 and the desired RF outputfrequency of the voltage-controlled oscillator 38, which oscillator inFIG. 2 may or may not include a crystal. Also provided in FIG. 2 is anA/D converter 60 (e.g., an 8-bit A;/D converter) which, based on signalsreceived from the microprocessor 12, is used to adjust the temperatureof thermal control system for the stable reference oscillator 32. FIG. 2allows for use of either a single stage thermal to control system (e.g.,a single oven), on a double stage thermal control system (e.g., a doubleoven).

In another embodiment according to the present invention, the bufferamplifier 34 can be augmented or replaced with a frequency doubler, toproduce a 10 MHz reference output signal, if desired. A stable referencevoltage 50 can be provided from the stable reference oscillator 32, suchas, a 10 V reference, or any other desired voltage.

The frequency-setting ability of an OXCO 10 according to the presentinvention is linear and predictable. In the preferred embodimentdescribed above, and for the exemplary values given in the drawing, thefrequency-setting ability of the OXCO 10 is calculated as:

Δƒ/bit=15MHz/2³²/2⁸=0.0001364Hz or

ƒ/bit/15 MHz=0.000013462 Hz/15 MHz=9.095⁻¹³/bit.

It is preferable to provide capability for adjusting the desired valueof the VCXO 38 output frequency. One method for accomplishing this isvia an analog input. As illustrated in FIG. 1, a 24-bitanalog-to-digital (A/D) converter 20 is provided, to which can beapplied a voltage ranging from 0 to 10V. The output signals of the A/Dconverter 20, or electronic frequency control (EFC), signals aredirected to the microprocessor 12 to cause a change in the control wordfor the DDS 16, to instruct the DDS 16 to synthesize a differentadjustment frequency. The ensuing signal from the DDS 16 and the divider26 will result in a new output of the first mixer 28 and the secondmixer 30. The result will be a change in the phase-lock frequency forthe phase-locked loop, and the RF output of the VCXO 38 thus will adjustto a new stable output frequency. In this manner, coarse or fineadjustments of the VCXO 38 output frequency can be accomplished.

A second method for accomplishing changes in the desired outputfrequency of the VCXO 16 is to communicate with the microprocessor 12over an RS232 digital interface 48. Imbedded software provided in themicroprocessor 12 can be used to interpret information input from theRS232 interface 48 and to reset the RF output of the VCXO 38 to a newadjusted output frequency that is different, for example, from the 15MHz output frequency indicated in FIG. 1.

In a preferred embodiment, a digital temperature sensor 44 isincorporated into the system to monitor the ambient operatingtemperature such that digital frequency compensation can be programmedin to the microprocessor 12 to additionally enhance accuracy andstability of the output frequency of the VCXO 38.

While it is preferable, in order to minimize phase disturbances, todivide the VCXO adjustment frequency down by divider 26 after it issynthesized by the DDS 16, it is also possible to divide the VCXO 38output frequency first, and then input the divider output into the DDSto create the VCXO adjustment frequency.

In accordance with one aspect of the invention, the VCXO 38 outputfrequency which is applied to RF amplifier 42 for use by other circuitryand the like, can be provided with an on/off control signal 46, toenable or disable the OCXO 10.

In keeping with still another aspect of the invention, themicroprocessor 12 and/or the memory 14 (e.g., E²P) can be programmedwith logic for furthering the stability of the OCXO 10. Such logic mayprovide compensation for temperature and for aging of other components,for example, the components of the device with which the OCXO 10 isemployed, such as a computer. As will be appreciated by those skilled inthe art, the programming may include algorithms and/or tables forproviding control signals to the DDS 16 for providing compensation overa predetermined range of values.

The principle method of thermal control in the OCXO 10 according to theinvention is, of course, via an oven or other thermal control system. Ina preferred embodiment, a double oven is used, which has a thermal gainof greater than 100,000. The outer oven is kept to a near zerotemperature gradient with a system of satellite heating semiconductorsthat are controlled by a main controller. The satellite heaters functionas point sources to provide uniform heat. Preferably, the same make andmodel of semiconductor heater is used for each of the point sources. Inone embodiment, a satellite heater is positioned on either side of anoven joint, so that the joint need not be optimized for heat transfer.The heat dissipation of the satellite heaters and the controller ispreferably is optimized over the outer oven surfaces in such a way as tominimize the temperature change of the crystal to less than 0.001° C.over an ambient temperature range of −55° C. to 75° C.

In accordance with one aspect of the present invention, the OCXO 10 canbe configured to output any RF output frequency in the range of 5 to 20MHz, for example, by replacing the 15 MHz VCXO 38 of the exampleillustrated in FIG. 1 with a VCXO having a desired RF output frequencyof from 5 MHz-20 MHz.

Although the invention has been described in language specific toparticular components of, or architecture for, a precision OCXO, it isto be understood that the invention defined in the appended claims isnot necessarily limited to the particular components or architecturedescribed. Therefore, the particular components and/or architecture aredisclosed as exemplary embodiments implementing the invention.

Further, the various embodiments described above are provided by way ofillustration only and should not be construed to limit the invention.Those skilled in the art will readily recognize various modificationsand changes that may be made to the present invention without followingthe example embodiments and applications illustrated and describedherein, and without departing from the true spirit and scope of thepresent invention, which is set forth in the following claims.Accordingly, the invention has been described by way of illustrationrather than limitation.

What is claimed is:
 1. A method of generating a stable and accurate RFoutput frequency from a voltage-controlled oscillator, comprising: (a)providing a voltage-controlled oscillator adapted to provide apredetermined RF output frequency; (b) providing a reference oscillatoradapted to provide a predetermined reference frequency; (c) providing afrequency synthesizer; (d) inputting the predetermined RF outputfrequency of the voltage-controlled oscillator into the frequencysynthesizer; (e) causing the frequency synthesizer to synthesize avoltage-controlled oscillator adjustment frequency; (f) mixing thevoltage-controlled oscillator adjustment frequency with the referencefrequency so as to provide a feedback signal; (g) inputting the feedbacksignal into a phase-locked loop so as to close the phase-looked loop andlock the voltage-controlled oscillator to the RF output frequency. 2.The method of claim 1, wherein providing a frequency synthesizercomprises providing a direct digital synthesizer.
 3. The method of claim1, wherein mixing the voltage-controlled oscillator adjustment frequencywith the reference frequency further comprises: (a) adding thevoltage-controlled oscillator adjustment frequency to thevoltage-controlled oscillator RF output frequency; (b) subtracting thesum of the voltage-controlled oscillator adjustment frequency and thevoltage-controlled oscillator RF output frequency from a multiple of thereference frequency.
 4. The method of claim 1, wherein causing thefrequency synthesizer to synthesize a voltage-controlled oscillatoradjustment frequency comprises causing the frequency synthesizer tosynthesize a voltage-controlled oscillator adjustment frequency that isderived from the value of the reference frequency.
 5. The method ofclaim 4, wherein causing the frequency synthesizer to synthesize avoltage-controlled oscillator adjustment frequency comprises causing thefrequency synthesizer in response to instructions from a microprocessorto synthesize a voltage-controlled oscillator adjustment frequency thatis derived from the value of the reference frequency.
 6. The method ofclaim 1, further comprising: (a) providing means for sensing thetemperature in the ambient environment of the voltage-controlledoscillator and the reference oscillator; (b) providing means foradjusting the synthesized frequency in response to the sensing means. 7.The method of claim 1, further comprising: (a) providing means forsensing the temperature in the ambient environment of thevoltage-controlled oscillator and the reference oscillator; (b)providing means for adjusting the temperature in response to the sensingmeans.
 8. The method of claim 1, further comprising: (a) providing meansfor changing the value of the predetermined RF output frequency of thevoltage-controlled oscillator.
 9. The method of claim 8, whereinproviding means for changing the value of the predetermined RF outputfrequency further comprises: (a) providing an analog-to-digitalconverter; (b) inputting at least one voltage signal into theanalog-to-digital converter to produce at least one electronic frequencycontrol signal; (c) inputting the at least one electronic frequencycontrol signal into the frequency synthesizer to cause the frequencysynthesizer to change the value of the frequency synthesized.
 10. Themethod of claim 8, wherein providing means for changing the value of thepredetermined RF output frequency further comprises: (a) providing adirect digital synthesizer; and (b) providing a microprocessor to changethe frequency output by the direct digital synthesizer.
 11. The methodof claim 9, further comprising: (a) adding the frequency synthesized tothe voltage-controlled oscillator RF output frequency; (b) subtractingthe sum of the frequency synthesized and the voltage-controlledoscillator RF output frequency from a multiple of the referencefrequency to create a feedback signal; (c) inputting the feedback signalinto the phase-locked loop so as to cause the RF output frequency toadjust to the changed value.
 12. A stable and accurate oscillator,comprising: a voltage-controlled oscillator capable of providing apredetermined RF output frequency; a reference oscillator capable ofproviding a predetermined reference frequency; a frequency synthesizercapable of providing an adjustment frequency for the voltage-controlledoscillator, based upon the predetermined reference frequency; aphase-locked loop, including the voltage-controlled oscillator; meansfor mixing the voltage-controlled oscillator adjustment frequency andthe reference oscillator reference frequency to create a feedbacksignal; means for introducing the feedback signal into the phase-lockedloop so as to lock the voltage-controlled oscillator to thepredetermined RF output frequency.
 13. The oscillator of claim 12,wherein the frequency synthesizer is a direct digital synthesizer. 14.The oscillator of claim 12, further comprising: a microprocessor forproviding instructions to the frequency synthesizer.
 15. The oscillatorof claim 12, wherein the mixing means further comprises: (a) a firstmixer into which the adjustment frequency and the RF output frequencyare input and added; (b) a second mixer into which the result of thefirst mixer and the reference frequency are input and the result of thefirst mixer is subtracted from a multiple of the reference frequency.16. The oscillator of claim 12, further comprising: means to sense andto provide a signal corresponding to the temperature in the ambientenvironment of the voltage-controlled oscillator and the referenceoscillator; and means to adjust the synthesized frequency, in responseto the sensing means, in the ambient environment of thevoltage-controlled oscillator and the reference oscillator.
 17. Theoscillator of claim 12, further comprising: a thermal control system tokeep constant the temperature in the ambient environment of thevoltage-controlled oscillator and the reference oscillator.
 18. Theoscillator of claim 12, further comprising: means to reset thepredetermined frequency of the voltage-controlled oscillator to adifferent value.
 19. The oscillator of claim 18, wherein the reset meanscomprises: (a) an analog-to-digital converter adapted to generate atleast one electronic frequency control signal based upon an inputvoltage, wherein the at least one electronic frequency control signal isinputted to the frequency synthesizer to cause the frequency synthesizerto change the value of the frequency synthesized.
 20. The oscillator ofclaim 18, wherein the reset means comprises: (a) a direct digitalsynthesizer; and (b) a microprocessor to provide instructions to changethe frequency output by the direct digital synthesizer.